Recently, CDMA (Code Division Multiple Access) system has become one of the mobile communication systems of a greater interest. CDMA system is a communication technology based on Spread Spectrum technology.
Generally, in a mobile communication environment, since a signal transmitted from a transmitter reaches to a receiver via a plurality of propagation paths, i.e., a so-called a multipath propagation path, a received signal is composed of a sum of multipath signals. Therefore, the received signal is composed of signal components having various time-of-arrivals, amplitudes and phases.
When a communication between a base station and mobile stations is based on CDMA, a so-called RAKE combining reception is possible, in which a signal received via a multipath propagation path is resolved into path components having different delay times and then combined after cophasing. Improved transmission characteristics of the RAKE combining reception may be achieved by improving a desired signal-to-power ratio against interference and thermal noise. Therefore, one of the most important technologies in the CDMA system is a path search method for detecting multipath timings with a considerably high accuracy for resolving into path components in a proper manner.
An example of a proposed prior art path search method may be found in an article “Path-Search Performance of DS-CDMA System in Laboratory and Field Experiments (Aoyama, Mizuguchi, Yoshida and Atokawa: The Technical Research Report of the Institute of Electronics, Information and Communication Engineers, RCS 97–164, pp. 51-58, November 1999)”.
According to this proposed path search method, timing detection of a path is implemented by performing a correlation calculation process, an averaging process of correlated values, and a peak detection process, using pilot symbols of a known phase which are periodically inserted in a received signal. In the correlation calculation process, in order to derive a symbol correlation value, a despreading process is performed by multiplying the pilot symbols of the received signal by a spread code. Further, based on the fact that the phase of the pilot symbols is known, the above-mentioned symbol correlation values are summed after cophasing, and then the values obtained from the summation after cophasing are power-summed for a fixed time duration.
Using a sequence of symbol correlation values (instantaneous delay profile) extracted by the above-described processes, a peak detection process is implemented for selecting paths available for RAKE combining. First of all, a path having the maximum level selected as a first path from the sequence of symbol correlation values. Then, as a second path, a path having the maximum level is selected from the symbol correlation values having a timing at a distance of more than at least r-chips of spread codes separate from the timing of the first path. Path selection is implemented in a similar manner for a third path and so on.
A further path search method of a prior art is, for example, proposed in an article, “Experiments on Path Search Performance of Coherent RAKE Receiver for W-CDMA Mobile Radio (Fukumoto, Ohkawa, Andoh, Sawahashi and Adachi: The Technical Research Report of the Institute of Electronics, Information and Communication Engineers, RCS 98-30, pp. 41–48, May 1998)”.
According to the proposed path search method, pilot symbols within a single slot are summed after cophasing to derive an instantaneous channel estimation value, and then the channel estimation values of successive two slots are cophased, summed and squared, so as to extract an instantaneous power delay profile. After extracting and averaging instantaneous power delay profiles of a plurality of slots, upper N paths having greater signal powers within the averaged power delay profile are regarded as a desired signal, and the power obtained by averaging the remaining paths excluding the upper N paths is assumed as a noise power Pn.
A power level of a factor of M of the noise power Pn is taken as a threshold value for path selection, and paths having signal powers exceeding this threshold are selected as paths of RAKE combining.
However, the above-mentioned path search method applies to a circuit-switched system in which, for a communication between mobile stations and a base station, signals continuously exist throughout a period from the start to the end of transmission.
Therefore, as in the case of signal transmission based on packets, in which the signals do not exist continuously but are transmitted intermittently, the above-mentioned path search method may give rise to a problem that an averaging process in a fixed period of time cannot be implemented and thus resulting a reduced path search accuracy.
Now, for a mobile communication system, a phenomenon called fading may occur due to a change in the relative position between a mobile station and a base station. Fading is a phenomenon in which an intensity of the received electric field temporally changes according to the state of a medium serving as a passage of an electric wave. Due to the fading phenomenon, the signals are received with their amplitude and phase being varied. Therefore, for an absolute coherent detection system in which information symbols are demodulated from absolute phase of the received signal, it is necessary to provide a method of accurately estimating the variation of amplitude and phase, i.e., a so-called channel variation, and compensating the channel variation.
Conventionally, as a channel estimation method for implementing absolute coherent detection, a method is proposed which uses pilot symbols having known phase. According to this channel estimation method, the pilot symbols having known phase are transmitted by being periodically multiplexed with the transmitted signals, and at the receiving end, the channel variation of the received signal is estimated using the pilot signals. Then, based on the result of the estimation, a channel variation of information symbols other than the pilot symbols is estimated. Generally, the channel variation of information symbols can be estimated by temporally interpolating the channel variation obtained from the periodically inserted pilot symbols.
For example, in the article “An Analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels” (J. K. Cavers: IEEE Transactions on Vehicular Technology, pp. 686–693, vol. 40, no. 4, November 1991)”, a method is proposed in which an amount of channel variation between pilot symbols is interpolated using a Wiener filter.
Also, in the article “Rayleigh Fading Compensation for QAM in Land Mobile Ratio Communications” (S. Sampei and T. Sunaga: IEEE Transactions on Vehicular Technology, pp. 1370147, vol. 42, no. 2, May 1993)”, a channel estimation method is proposed in which a low-level Gaussian interpolation is used for interpolation. Other methods, such as those using linear interpolation, are also proposed.
Also, in order to improve an accuracy of channel estimation, a method is proposed in which an absolute coherent detection is implemented using only the pilot symbols, and the tentative data decision information symbols are remodulated and fed back. After that, the received signals are multiplied by the complex conjugate of the fed-back symbols, and modulation components are removed to generate non-data modulated information symbols, and these symbols as well as the pilot symbols are both used for implementing channel estimation in a repeated manner.
Such a method is, for example, described in “Symbol-Aided Plus Decision-Directed Reception for PSK/TCM Modulation on Shadowed Mobile Satellite Fading” (G. T. Irvine and P. J. McLane: IEEE Journal on Selected Areas in Communications, pp. 1289–1299, vol. SAC-10, October 1992)”.
Also, in order to reduce the data decision error of the tentative data decision information symbols, a method is known in which the information symbols are performed after error correction decoding process. In this case, tentative data decision is implemented after absolute coherent detection using only the pilot symbols and after an error correction decoding process.
For example, such a method is described in “Performance of Coherent Detection with Decision Feedback Interpolation and Viterbi Decoding on DS/CDMA” (Azuma, Taguchi and Ohno: The Proceedings of the 1994 Autumn Conference of the Institute of Electronics, Information and Communication Engineers, B-305”.
However, the above-mentioned channel estimation method using pilot symbols is aimed for use in a situation where channels are always assigned by a circuit-switched system during a communication between a mobile station and a base station and signals are continuously transmitted and received.
However, with a packet wireless access system in which information symbols are transmitted/received in a format called packets, signals are intermittently transmitted and received during the communication between a mobile station and a base station. That is to say, the pilot symbols cannot be periodically multiplexed as in the case of the circuit-switched system.
Also, with the above-mentioned channel estimation method which uses both the pilot symbols and the information symbols wherefrom the modulation components are removed, the tentative data decision information symbols are remodulated and are all fed back. However, in a mobile communication system, since the reliability of the received signal varies due to noise, interference signals, etc., it is not preferable to remodulate the tentative data decision information symbols and feedback all of them.